In recent years, electric utilities have discovered the desirability of leveling or reducing their peak load energy usage, especially during summer months. Concurrent with the development of the present invention, a long term energy storage system has been proposed in which a prime mover, such as an ice machine, is operated on a continuous basis for a period of months, or even year-round. In the operation of such a system, the ice is stored for use on demand during periods that coincide with the electric utility's seasonal peak load operating times in the summer, for example, to provide space air conditioning, harvested crop cooling, greenhouse space cooling, poultry house cooling, etc. This system enables the electric utility to substitute a relatively small long term level load for a much higher direct acting load that would coincide with or largely overlap the utility's seasonal peak usage. Further, the customer benefits by purchasing a significant amount of his electricity at a less expensive, "off peak" or "load levelled" rate.
The above system that utilizes an annual ice thermal storage cooling system is discussed in detail in a commonly assigned application Ser. No. 543,169, filed on even date herewith, entitled "Long Term Thermal Energy Production, Storage and Reclaim System", now U.S. Pat. No. 5,046,551. A primary purpose of the present invention is to provide a storage tank for long term storage of the ice generated in the operation of the system described in the co-pending application.
Significant technology exists in the area of above ground storage tanks for fluids. Some of these tanks incorporate thermal insulation. These tanks could, with significant modification, adequately serve as the thermal storage tank for ice storage systems of the type discussed above. However, the cost, space requirements and unsightliness of above ground storage tanks render this approach impractical for long term ice storage located at commercial properties, residences, farms, etc.
Earth-excavated storage tanks have been found to offer substantial promise for long term thermal storage needs. First, these tanks can be substantially or totally hidden from view. Second, earth-excavated tanks generally are the least expensive volumes for storing large volumes of fluid.
The most common proposals for earth-excavated thermal storage tanks take the form of double wall structures wherein the two walls are separated by an air space. However, the expense of double wall construction is high, and in some cases exorbitant. Also, unless the outer wall is impervious to ground water seepage, the space between the walls must be provided with mean for discharging water that fills this space, for example, by a sump pump.
A double wall earth-excavated storage tank having solid insulating material between the two walls is described in U.S. Pat. No. 4,183,221. Another type of double wall earth-excavated tank having incompressible insulating material between a pair of thin plastic walls is shown in U.S. Pat. No. 4,011,736.
Thus, the construction of a single wall thickness earth-excavated thermal storage tank would be desirable to avoid the complexities and expense of double wall structures. However, the wall material of such a tank must provide sufficient strength, durability and long term thermal insulating characteristics even when exposed for years to the earth and groundwater. Conventional insulating materials such as fiberglass, mineral wood and cellulose meet none of the above requirements. These materials must be layered in conjunction with the structural components of the tank and kept dry by liners or other means. Other materials, such as polystyrene, polyurethane and rigid fiberglass are marginal in terms of durability and strength and are absolutely unsatisfactory from a long term thermal insulating standpoint since they lose their thermal insulating properties when wet. It is known that conventional concrete provides adequate strength and durability, but conventional concrete is a relatively poor thermal insulator when dry, and even worse when wet. Finally, concrete materials containing various insulating materials have been proposed for use in the construction industry for thermal insulation. These concretes, which use vermiculite, ground rigid polystyrene, or other similar materials as the aggregate, provide adequate insulation but have poor strength, thus rendering them inappropriate as a substitute for structural concrete. Furthermore, these concrete materials lose their thermal insulating properties when they become wet. Most recently, studies conducted on behalf of Oak Ridge National Laboratories of Oak Ridge, Tenn. have been directed to the development of a thermally insulating concrete having a moderate strength so that it can be used as the wall component of low rise buildings.
None of the above materials meet the requirements for a single wall thickness earth excavated storage tank Thus, the provision of long term energy storage in such tanks requires an advancement that will permit tank construction at reasonable cost, and provide the required properties stated above.
Turning to another aspect of the present invention, in thermal storage tanks that store ice in an ice/water mixture there is a need for an efficient manner and means for introducing ice into the tank to achieve a stable, buoyant iceberg shape. Also, there is no known system for matching the manner of introducing the ice into the mixture with the geometry of the tank. It would be desirable to match these system features to make the thermal storage more efficient and to minimize "dead space" in the tank. Furthermore, it is desirable to provide a ballast to the ice mass in the tank to maintain the ice mass in a submerged state to prevent sublimation losses.